Device and method for linear illumination of an object using leds and an elliptical mirror

ABSTRACT

A device, method and system is provided for illuminating an object. A light source such as an LED and in particular a polychromatic LED is positioned at a first focal line of an elliptical mirror that has two focal lines. The mirror is shaped as a portion of an inner generated surface of an elliptical cylinder. The object to be illuminated is positioned at the second focal line of the mirror. A plurality of such mirrors may be provided positioned with a common second focal line. A cascade of mirrors may be positioned with a common second focal line at a first focal line of a concentrating mirror. The LEDs may be mounted on one or both sides of a circuit board.

[0001] The invention concerns a device to linearly illuminate an object. Furthermore, the invention concerns a method to linearly illuminate an object and a scanning system.

[0002] A typical use for a device to linearly illuminate an object is an image acquisition system, in which a sensor (for example a camera) detects graphical elements of a master, digitalizes these graphical elements, and processes the image data. The quality of an image acquisition system is directly dependent on the amount of light that is available to the sensor during the scanning to generate the corresponding signals. It is therefore desirable (in particular in the field of rapid line-scanning systems) to provide a large amount of light, since a higher signal-to-noise ratio is thereby supplied for the subsequent signal processing. Furthermore, it is necessary to employ a light source with as little power dissipation as possible that exhibits a high time stability and temperature stability.

[0003] Light-emitting diodes, what are known as LEDs, have very good characteristics for use in image acquisition system with regards to lifespan, time stability, temperature stability, and efficiency of the light yield. Known scanning systems that use LED lines focus the radiation with the aid of bar lenses or Fresnel lenses. Such systems are used for monochromatic light.

[0004] Due to newer developments, polychromatic LEDs are currently available that also emit white light. However, such LEDs have a smaller light-power density, such that they can only be employed in a very restricted manner in image acquisition systems. The use of lens components, for example of bar lenses or Fresnel lenses, in polychromatic light leads to significant chromatic image errors. These chromatic image errors manifest themselves in color errors on the focus edges. However, in the scanning of color graphic image elements it is necessary to have a homogenous color temperature for the entire field of illumination in order to prevent an incorrect scan. Therefore, it is only possible with great effort to focus light radiation of polychromatic LEDs with the aid of lens systems.

[0005] An optical scanning system is known from U.S. Pat. No. 5,828,050 that operates with a curved pivotable mirror. The radiation of a monochromatic LED is directed by the mirror as a light spot on to an object to be scanned. The received beam path of a light-receiving diode is directed via the same mirror, such that the graphical elements illuminated in the light spot are scanned in the pivoting movement of the mirror.

[0006] A scanning system with a mirror arranged in stationary fashion is specified in U.S. Pat. No. 5,412,205 that is elliptical in cross-section. An elongated fluorescent tube is arranged in the first focal line of the mirror, the document to be scanned in the second focal line. The radiation of the fluorescent tube is focused by the mirror linearly onto the document.

[0007] An image reading device is known from JP 08-307610A that uses an LED as a light source. This LED has an elliptical mirror, whereby the LED is arranged in a focal point of the mirror and the other focal point of the mirror is arranged on the image to be read.

[0008] Furthermore, reference is made to the following prior art: JP 2000-312 305 A, FR 1 237 467, DE-OA 2 217 421, U.S. Pat. No. 6,133,565, JP 56-106 257 A with abstract, DE 195 32 877 A1, DE 42 23 916 C2. The use of elliptical mirrors in connection with digital cameras and scanning devices is specified in these documents.

[0009] Document U.S. Pat. No. 4,287,414 forms the preamble for the independent claims.

[0010] It is the object of the invention to specify a device to linearly illuminate an object that has a compact assembly and has a large quantity of illumination light available.

[0011] This object is achieved for a device according to the features of claim 1. Advantageous developments are the subject matter of the dependent claims.

[0012] According to the invention, a mirror arranged in stationary fashion is used to focus radiation. This mirror has two focal lines. A plurality of LEDs are arranged along one focal line. The radiation detected from the mirror is focused in the second focal line.

[0013] Due to the use of LEDs, the device has a compact assembly as well as further advantageous characteristics with regards to lifespan of the light source, time stability, temperature stability, and light yield. Due to the use of a stationary mirror, the beam path remains clear, since the position of the two focal lines of the mirror are given high degrees of design freedom. Due to the use of the mirror, no wavelength-dependent chromatic errors arise, such that no color errors ensue at the focus edges given use of LEDs that emit white light. This is advantageous when the device to linearly illuminate an object is comprised in a scanning system in which colored image elements should be scanned by means of a linearly operating scanning system, for example in digital scanner systems or copier systems.

[0014] According to the invention, the cylindrical section of the mirror has the form of the inner generated surface of an elliptical cylinder. The elliptically-shaped mirror has the property to fully focus radiation that is emitted from the first focal point of the ellipse onto the second focal point. Due to the geometric reflection of the mirror surface, no chromatic errors occur. Therefore, such a mirror system is well appropriate for polychromatic light. Given use of an elliptical cylinder, a focal line arises instead of a focal point, whereby an optimal linear illumination of an object is enabled. This linear illumination is particularly advantageous when an object must be digitally scanned.

[0015] According to further aspects of the invention, a method is specified under use of the cited device. Further specified is a system included in a sensor system, for example a camera, to scan an object.

[0016] Exemplary embodiments of the invention are explained in the following using the drawings. Thereby shown are:

[0017]FIG. 1 an arrangement of an LED on a circuit board, and the radiation characteristic of the LED,

[0018]FIG. 2 the principle of the radiation reflection on an elliptical mirror,

[0019]FIG. 3 an arrangement with a plurality of elliptical mirrors and common focal points or, respectively, focal lines,

[0020]FIG. 4 an embodiment with LED lines arranged double-side on a circuit board,

[0021]FIG. 5 an illumination device with an LED row,

[0022]FIG. 6 a symmetric arrangement of two LED lines and two elliptical mirrors,

[0023]FIG. 7 the cross-section of a mirror in the form of an elliptical cylinder generated surface, and

[0024]FIG. 8 a perspective representation of a fashioned sheet serving as an elliptical cylinder mirror.

[0025]FIG. 1 shows the arrangement of an LED 10 on a circuit board 12. This circuit board 12 carries conductor paths and, if necessary, also electronic components to activate the LED 10. This LED 10 emits polychromatic light, in particular white light. The generation of polychromatic light is possible due to the use of fluorescing conversion materials in the semiconductor assembly of the LED. What is problematic is that the radiated power density is relatively small in such polychromatic LEDs 10. Given the use of polychromatic LEDs for scanning systems or line-scanning systems with high requirements in terms of the amount of light, an efficient focusing of the emitted radiation is therefore necessary. It is shown in FIG. 1 that the radiation, indicated by arrow 14, ensues in the half-space according to type of a spherical light dispersal curve 16.

[0026]FIG. 2 shows the principle used in the invention. If a polychromatic LED 10 that emits light according to the light dispersal curve 16 is arranged in the focal point 20 in an elliptical mirror 18 with the two focal points 20, 22, then this light is reflected on the elliptical mirror and focused in the second focal point 22. In principle, this is true for all beams. However, this is only utilized for beams that are emitted in the half-space in the direction of the light dispersal curve 16; theoretically, the beam 24 is also focused in the focal point 22. In general, it is sufficient to use only a section of the ellipse 18 as a mirror in order to achieve a sufficient concentration of the radiation emitted in the half-space. By skillful selection of the ellipse parameters, a compact geometric arrangement with many degrees of design freedom can be achieved. Due to the geometric reflection on the elliptical surface of the mirror 18, no chromatic errors arise; rather, the beams of different wavelengths are focused in the focal point 22 without the color edges arising.

[0027] In order to achieve a linear illumination, a section of an inner generated surface of an elliptical cylinder is used as a mirror. The focal points 20, 22 are then points on focal lines 20, 22. If a plurality of LEDs 10 are arranged in the focal line 20, a light line arises in the focal line 22 that is particularly well-suited for the optoelectronic scanning of masters.

[0028]FIG. 3 now shows a expansion of the principle shown in FIG. 2 via parallel focusing with the aid of a plurality of elliptical mirrors 26, 28, 30, whereby again mirror sections are sufficient for reflection of the radiation. In the respective first focal points or, respectively, focal lines 32, 34, 36, one or, respectively, more LEDs according to type of the LED 10 are arranged. The various mirrors 26, 28, 30 have a common second focal point 38 or, respectively, a common focal line 38. This focal point 38 or, respectively, focal line 38 is again a first focal point 38 or focal line 38 of a further elliptical concentrating mirror 40 with a second focal point 42 or, respectively, focal line 42 when the concentrating mirror 40 is fashioned as an elliptical cylinder mirror.

[0029] On the assumption that the mirrors 26, 28, 30, 40 have elliptical cylinder mirror sections, the radiation starting from the first focal lines 32, 34, 36 is focused in the second focal line 38 common to all mirrors. The radiation starting from this focal line 38, that coincides with the first focal line 38 of the mirror 40, is focused in the second focal line 42 of the mirror 40. Due to the series connection of the different mirrors, it is also possible to increase the light density still further in the focal line 42. The cascading with further mirrors can be continued, in that the focal line 42 is the first focal line of a further concentrating mirror (not shown). The radiation is then imaged in this second focal line.

[0030] In the case that not a light line, but rather a light spot, should be generated, a rotation element is to be used as a mirror, for example a section of an ellipsoid. Such a mirror then has no focal lines, but rather focal points. The radiation originating from the focal points 32, 34, 36 is then focused on the ellipsoid mirror surfaces of the mirrors 26, 28, 30 in the focal point 38 that coincides with the first focal point of the ellipsoid concentrating mirror 40. The radiation starting from the focal point 38 is then focused in the focal point 42, where a light spot of higher radiation density arises.

[0031]FIG. 4 shows an arrangement of a device for the linear illumination of an object O, whereby identical parts are subsequently identically designated. 12 LEDs 10 are arranged on a circuit board 12 in a row on both sides of the circuit board 12. These LEDs 10 are arranged along the first focal line of two elliptical cylinder mirrors 46, 48. These mirrors 46, 48 focus the radiation in their (respectively) second focal line 50 that topically coincides and illuminates the object O. The shown device has a compact assembly, since the emission characteristic of the LED that only emits radiation in a half-space is linked with the beneficial imaging characteristics of the elliptical mirror 46, 48. In this manner, it is possible to detect the total emitted radiation with the aid of a relatively small mirror element. By selecting the ellipse parameter, a largely more compact assembly can be achieved, such that radiation can be also focused on the object O over a relatively large distance.

[0032]FIG. 5 shows an assembly with only one row of LEDs 10 on the circuit board 12. The elliptical mirror 52 is directly connected to the circuit board 12, whereby a simpler design assembly arises. The object O to be illuminated is tilted relative to the vertical in which the circuit board 12 lies. The object O linearly illuminated is scanned with the aid of a camera (not shown) from the direction 54.

[0033]FIG. 6 shows an assembly symmetric to an axis of symmetry 56 that is perpendicular to the object O, for example a printing master. The axis of symmetry 56 also simultaneously forms the middle axis of the angular field for a scanning camera. The elliptical mirrors 58, 60 respectively form a unit with the circuit board 12. The light line originating on the object O can freely be accessed from the direction 56. A cast shadow can be largely avoided in the shown arrangement.

[0034]FIG. 7 shows the assembly of the elliptical mirror cylinder 62 in cross section. It is produced by forming a sheet, for example with the thickness 0.5 mm.

[0035]FIG. 8 shows a perspective representation of the elliptical mirror cylinder 62, on whose edges attachment holes 64 are provided.

[0036] The shown devices can be used to particular advantage for image scanning systems, digital scanners, and copier systems.

[0037] Reference List

[0038]10 LED

[0039]12 circuit board 12

[0040]14 radiation arrow

[0041]16 light dispersal curve

[0042]18 elliptical mirror

[0043]20, 22 focal points, focal lines

[0044]24 beam

[0045]26, 28, 30 elliptical mirror

[0046]32, 34, 36 focal lines, focal points

[0047]38 second focal point, second focal line

[0048]40 concentrating mirror

[0049]42 second focal point, second focal line

[0050]46, 48 elliptical cylinder mirror

[0051]50 second focal line

[0052] O object

[0053]52 elliptical mirror

[0054]54 direction axis

[0055]56 axis of symmetry

[0056]58, 60 elliptical mirror

[0057]62 elliptical cylinder mirror made from a sheet

[0058]64 attachment holes 

1. Device to for the linear illumination of an object (0) [sic], in that at least one mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) arranged in a stationary fashion has a concave curved elongated cylindrical section and two focal lines (32, 38; 34, 38; 36, 38; 38, 42; 50), a plurality of LEDs (10) are arranged along a focal line (32, 34, 36) that emit radiation in the direction of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62), and in that the emitted radiation is concentrated in the second focal line (38, 42, 50).
 2. Device according to claim 1, in that the cylindrical section of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) has the form of the inner generated surface of an elliptical cylinder.
 3. Device according to any of the preceding claims, in that a plurality of mirrors (26, 28, 30) with respectively a first focal line (32, 34, 36) and a second focal line (38) are provided, for each mirror (26, 28, 30), a plurality of LEDs (10) are arranged along the first focal line (32, 24, 36) that emit radiation in the direction of the respective mirror (26, 28, 30), and in that the radiation reflected from the plurality of mirrors (26, 28, 30) is concentrated in a common second focal line (38).
 4. Device according to claim 3, in that a concentrating mirror (40) with two focal lines (38, 42) is provided, whose first focal line (38) coincides with the cited common focal line (38), and in that the radiation coming from the first focal line (38) is concentrated in the second focal line (42) of the concentrating mirror (40).
 5. Device according to claim 4, in that a cascading with a plurality of concentrating mirrors with respectively two focal lines is provided, in that a concentrating mirror positioned in front concentrates in its second focal line the radiation originating from its first focal line, and in that the first focal line of the concentrating mirror positioned behind coincides with the second focal line of the concentrating mirror positioned in front, and concentrates the radiation in its second focal line.
 6. Device according to any of the preceding claims, in that the LEDs (10) generate polychromatic light.
 7. Device according to claim 6, in that the LEDs emit visible light, preferably white light.
 8. Device according to any of the preceding claims, in that the object (0) to be illuminated is arranged along the second focal line (22; 38) of the mirror (18) or of the plurality of mirrors (26, 28, 30) or, given a cascading, in the second focal line (42) of the concentrating mirror (40) positioned last.
 9. Device according to any of the preceding claims, in that the LEDs (10) are arranged in a row on a circuit board (12), and in that the mirror (52) is arranged near the circuit board (12) or connected to it.
 10. Device according to any of the preceding claims, in that a circuit board (12) is equipped on both sides with a row of LEDs (10) per side, and in that on each side of the circuit board (12) a mirror (46, 48) is arranged whose second focal lines meet in a concentrating focal line.
 11. Device according to any of the preceding claims in that two circuit boards (12) with LEDs (10) and a mirror (58, 60) per each are arranged symmetric to an axis of symmetry (56), the second focal lines of the mirrors (56, 58) meet in a concentrating focal line on the axis of symmetry (56), and in that the object (0) to be illuminated is arranged in this concentrating focal line.
 12. Device according to any of the preceding claims, in that a camera scans the illuminated object (0).
 13. Method for linear illumination of an object (0), in that at least one mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) arranged in a stationary fashion has a concave curved elongated cylindrical section and two focal lines (32, 38; 34, 38; 36, 38; 38, 42; 50), a plurality of LEDs (10) are arranged along a focal line (32, 34, 36) that emit radiation in the direction of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62), and in that the emitted radiation is concentrated in the second focal line (38, 42, 50).
 14. Method according to claim 13, in that the cylindrical section of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) has the form of the inner generated surface of an elliptical cylinder.
 15. Method according to any of the preceding claims, in that a plurality of mirrors (26, 28, 30) are used with a first focal line (32, 34, 36) and a second focal line (38), for each mirror (26, 28, 30), a plurality of LEDs (10) are arranged along the first focal line (32, 24, 36) that emit radiation in the direction of the respective mirror (26, 28, 30), and in that the radiation reflected from the plurality of mirrors (26, 28, 30) is concentrated in a common second focal line (38).
 16. Method according to claim 15, in that a concentrating mirror (40) with two focal lines (38, 42) is used, whose first focal line (38) coincides with the cited common focal line (38), and in that the radiation coming from the first focal line (38) is concentrated in the second focal line (42) of the concentrating mirror (40).
 17. Method according to claim 16, in that a cascading with a plurality of concentrating mirrors with respectively two focal lines is provided, in that a concentrating mirror positioned in front concentrates in its second focal line the radiation originating from its first focal line, and in that the first focal line of the concentrating mirror positioned behind coincides with the second focal line of the concentrating mirror positioned in front, and concentrates the radiation in its second focal line.
 18. Method according to any of the preceding claims, in that the LEDs (10) generate polychromatic light.
 19. Method according to claim 18, in that the LEDs emit visible light, preferably white light.
 20. Method according to any of the preceding claims, in that the object (0) to be illuminated is arranged along the second focal line (22; 38) of the mirror (18) or of the plurality of mirrors (26, 28, 30) or, given a cascading, in the second focal line (42) of the concentrating mirror (40) positioned last.
 21. Method according to any of the preceding claims, in that a camera scans the illuminated object (0).
 22. System for linear illumination of an object (0), in that at least one mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) arranged in a stationary fashion has a concave curved elongated cylindrical section and two focal lines (32, 38; 34, 38; 36, 38; 38, 42; 50), a plurality of LEDs (10) are arranged along a focal line (32, 34, 36) that emit radiation in the direction of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62), in that the emitted radiation is concentrated in the second focal line (38, 42, 50) in which the object (0) is arranged, and in that a sensor detects the radiation radiated from the object (0).
 23. Device for the illumination of an object (0) by a light spot, in that at least one mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) arranged in a stationary fashion has a concave curved section and two focal points (32, 38; 34, 38; 36, 38; 38, 42; 50), a plurality of LEDs (10) are arranged at a focal point (32, 34, 36) that emit radiation in the direction of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62), and in that the emitted radiation is concentrated in the second focal point (38, 42, 50).
 24. Device according to claim 23, in that the cylindrical section of the mirror (26, 28, 30, 40, 46, 48, 52, 58, 60, 62) has the form of the inner generated surface of an ellipsoid.
 25. Device according to any of the preceding claims, in that a plurality of mirrors (26, 28, 30) with respectively a first focal point (32, 34, 36) and a second focal point (38) are provided, for each mirror (26, 28, 30), at least one LED (10) is arranged at the first focal point (32, 24, 36) that emits radiation in the direction of the respective mirror (26, 28, 30), and in that the radiation reflected from the plurality of mirrors (26, 28, 30) is concentrated in a common second focal point (38).
 26. Device according to claim 35 [sic], in that a concentrating mirror (40) with two focal points (38, 42) is provided, whose first focal point (38) coincides with the cited common focal point (38), and in that the radiation coming from the first focal point (38) is concentrated in the second focal point (42) of the concentrating mirror (40).
 27. Device according to claim 26, in that a cascading with a plurality of concentrating mirrors with respectively two focal points is provided, in that a concentrating mirror positioned in front concentrates in its second focal point the radiation originating from its first focal point, and in that the first focal point of the concentrating mirror positioned behind coincides with the second focal point of the concentrating mirror positioned in front, and concentrates the radiation in its second focal point.
 28. Device according to any of the preceding claims, in that the LEDs (10) generate polychromatic light.
 29. Device according to claim 28, in that the LEDs emit visible light, preferably white light.
 30. Device according to any of the preceding claims, in that the object (0) to be illuminated is arranged at the second focal point (22; 38) of the mirror (18) or of the plurality of mirrors (26, 28, 30) or, given a cascading, in the second focal point (42) of the concentrating mirror (40) positioned last.
 31. Device according to any of the preceding claims, in that the LED (10) is arranged in a row on a circuit board (12), and in that the mirror (52) is arranged near the circuit board (12) or connected to it.
 32. Device according to any of the preceding claims, in that a circuit board (12) is equipped on both sides with an LED (10) per side, and in that on each side of the circuit board (12) a mirror (46, 48) is arranged whose second focal points meet in a concentrating focal point.
 33. Device according to any of the preceding claims in that two circuit boards (12) with LEDs (10) and a mirror (58, 60) per each are arranged symmetric to an axis of symmetry (56), the second focal points of the mirrors (56, 58) meet in a concentrating focal point on the axis of symmetry (56), and in that the object (0) to be illuminated is arranged in this concentrating focal point.
 34. Device according to any of the preceding claims, in that a camera scans the illuminated object (0). 